Systems of concentric, alternately light and dark rings are as a rule used as the pattern. They can be produced in various ways. Thus, it is, for example, possible to mount reflecting ring marks inside of a hollow spherical or paraboloidally shaped housing, on which the light from a light source provided in the optical axis is reflected, as this is, for example, already described in the Patent DE 32 33 178 C2. Such a device demands a considerable amount of industrial input: The ring marks must be constructed with extraordinary precision and must be arranged in the housing in order to produce a regular pattern. Small errors in the position of the reflecting surfaces on the ring marks result in considerable distortions of the pattern. The light source must be mounted rather exactly in the focal point of the concentrating reflector formed by the ring marks if the pattern is supposed to be evenly illuminated.
An arrangement of such a ring mark system, which arrangement instead is provided in a hollow-cone-shaped housing, is described in the Patent Application EP 0 589 857 A1. The housing is transparent throughout and is illuminated on the back side of the ring marks by an annular light source. The light source surrounds the housing coaxially on its front side with the small diameter, in the vicinity of which in addition a fixation point in the beam path can be reproduced, which is produced by means of a light diode and makes possible during the measurement of a cornea of an eye a (reproducible) at-rest position of the eye. The light source thereby demands a considerable energy supply without assuring a uniform illumination of the pattern. Small form and/or position errors of the ring marks are also in this arrangement disadvantageous for the uniformity of the pattern.
It is known from the German Offenlegungsschrift DE 43 25 494 A1 to construct the image pattern with the help of a hollow truncated-cone-shaped screen aperture, in which the annular aperture elements alternate with annular gaps. The screen aperture is illuminated by a surrounding light source and produces also a pattern of light and dark rings on the spherical surface, which is to be measured and is provided in its axis and in front of the larger front side. Such a screen aperture can be manufactured relatively easily and with precision so that the pattern is produced with a good regularity without much input and/or expense.
A pattern, which is reproduced in this manner, for example, on the cornea of an eye, can be measured with the help of a sensory mechanism; a video camera is, for example, suitable for this purpose.
The measurement results, which are evaluated by means of a suitable evaluating digital unit, are readied by the sensory mechanism and are, if desired, compared with a pattern. The measurement results are used, for example, to fit contact lenses or to guide surgical instruments during the surgical correction of curvature radii if the cornea of an eye was measured with the device.
Fluorescent lamps formed in most cases into a circular ring are used as light sources, which lamps demand much space and emit much heat during operation. Their glare is undesired during the measuring of the cornea of the eye.
A device for the topographical measurement of a surface is known from EP 0 397 962, in which the individual light spots on the surface of a hollow conical housing are produced by a plurality of light-emitting diodes, which directly and without additional illumination of a screen aperture produce a light pattern.